Add docs quality gates for Markdown changes

[fabthebest]

Summary

• add a repo-wide GitHub Actions workflow that runs on Markdown changes in pull requests and on pushes to main
• lint only the Markdown files changed in a PR to avoid blocking contributors on unrelated pre-existing docs debt
• validate external links for those changed Markdown files with a shared lychee configuration
• add shared root configuration for markdownlint-cli2 and lychee
• document the local docs quality workflow in CONTRIBUTING.md and link to it from the README

Why

This addresses the lack of automated docs quality gates in the repository and reduces the risk of inconsistent Markdown styling or broken external links slipping into merged changes.

Validation

• ran markdownlint-cli2 locally on the edited Markdown files
• verified the external links referenced in README.md and CONTRIBUTING.md resolve successfully

Closes #2005

Drafting assistance from Codex; workflow design, testing, and final review done by me.

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